1. Field of the Invention
The present invention relates to the field of generators. It relates to a generator which is operated according to the induced-draft-cooling principle and in which a main fan, attached to the rotor shaft of the generator, draws cooling medium, which is heated by the heat-generating elements of the generator, out of the generator and transports it essentially perpendicularly to the axis of this rotor shaft to a cooling arrangement, the drawn-in cooling medium, which is expelled with a swirl at high velocity by the main fan, being received by a cooling-duct casing, which directs the cooling medium to the cooling arrangement via at least one fluidic constriction.
2. Discussion of Background
In generators which are operated according to the induced-draft-cooling principle, the cooling medium heated by the heat-generating elements of the generator is drawn out of the generator by main fans fastened to the rotor shaft. These main fans are normally arranged at the front end and rear of the generator, and a duct system directs the cooling medium blown out by the main fans to a cooling arrangement, which usually lies under the generator, in a foundation pit. Heat is then extracted from the cooling medium when flowing through the cooling arrangement, extending essentially over the entire length of the generator, and the cold medium, in a ducted manner over the entire length, thereupon passes into the interior of the generator again to the heatgenerating elements, hence forming a closed cooling circuit.
In order to meet the present requirements for the cooling of rotors in limit-rating machines, increasingly efficient and increasingly loss-free ducting of the cooling-medium flows is indispensible. A critical location for the cooling-medium flows is in the head-side and front-end regions, where the main fans draw the heated cooling medium out of the generator interior space. There, the hot cooling medium enters a cooling duct with a high swirl and at a high velocity, the task of this cooling duct being to duct the cooling medium downward to the cooling arrangement. So as not to unnecessarily increase the generator length, this cooling duct is to be kept as narrow as possible. The flow drawn out of the generator interior space by the main fan can be split into two components. The cooling medium flows out of the generator interior space parallel to the rotor shaft; in addition, however, the cooling medium is also set in rotation by the main fan. The cooling duct deflects this swirled flow perpendicularly to the rotor shaft and collects the flow, fanned out perpendicularly to the rotor shaft in all directions, in one direction, namely downward.
In this case, the deflection of the cooling-medium flow into the circular-symmetrical fantail perpendicularly to the rotor shaft is usually assisted by a so-called plate diffuser. This plate diffuser is essentially a tube which encloses the main fan and opens in a trumpet shape downstream of the fan in the direction of flow to form a plane circular ring lying perpendicularly to the rotor shaft and parallel to the front end of the generator. As a result, the cooling medium is ducted directly at the main fan and is then fanned out in a controlled manner. The cooling-duct casing is normally designed in the same way as the casing of the generator and encloses the cooling duct essentially in a circle, leaving open that side which leads into the foundation pit, so that the cooling medium can flow away downward.
A problem with this ducting of the cooling-medium flow, as things stand, is that at least one constriction occurs in the cooling-medium flow. This is because the entire cooling-medium flow expelled upward and laterally by the fan must, since it is swirled, flow through one zone in the cooling duct. In this zone, the constriction, which lies at the bottom just upstream of the opening of the cooling-duct casing in the direction of flow, vortices and flow separations of the cooling medium occur as a result of the high velocities. The pressure drops associated therewith reduce the efficiency of the rate of flow of the cooling medium in an unacceptable manner.